A ceramic honeycomb structure (simply called “honeycomb structure” below) used as a filter for cleaning an exhaust gas, etc. is produced by extruding a moldable ceramic material through a honeycomb-structure-molding die (simply called “molding die”) to form a honeycomb molding, and drying and sintering it. As shown in FIGS. 2(a) and 2(b), the molding die 10 is formed from a die-forming work 11 having such a shape that a groove-having surface 21 projects, that moldable-material-supplying apertures 30 are formed in the die-forming work 11 such that they extend from an aperture-having surface 31, and that lattice-patterned grooves 20 are formed on a groove-having surface 21. As shown in FIG. 3, the moldable-material-supplying apertures 30 of the molding die 10 are communicating with the molding grooves 20. A moldable ceramic material introduced into the molding die 10 through the apertures 30 are formed into a honeycomb shape by the grooves 20 to provide a honeycomb molding.
The molding die 10 is produced by forming apertures 30 by drilling, etc. in the die-forming work 11 having a projecting groove-having surface 21 as shown in FIG. 4(a) from its aperture-having surface 31 (on the opposite side of the groove-having surface 21), and then forming the grooves 20 on the groove-having surface 21. The lattice-patterned grooves 20 are produced by forming pluralities of first parallel grooves 20a by grinding or cutting by a rotating tool 40 (first machining operation) as shown in FIG. 4(b), rotating the die-forming work 11 by 90°, and then forming pluralities of second parallel grooves 20b crossing the previously machined grooves 20a (second machining operation) as shown in FIG. 4(c).
When a narrow, deep grooves 20 are formed by a rotating thin tool 40, the tool 40 is likely deformed or warped by large machining resistance, resulting in meandering grooves and damaging of the tool 40.
Accordingly, grooves 20 having large depth relative to width are usually formed by 2 or more passes. Namely, first-pass machining is conducted to form a groove 22 having depth L1 as shown in FIG. 6(a), and second-pass machining is conducted to form a groove 20 having depth L, which is deeper than the groove 22, as shown in FIG. 6(b).
Conventional machining of grooves 20 is conducted by so-called down-cutting, in which the tool 40 is rotated in the direction shown in FIG. 4(b). If machining were conducted with the rotation direction of the tool 40 reversed from that shown in FIG. 4(b), namely by up-cutting, a force in a direction of lifting the die-forming work 11 is likely to cause micro-vibration, so-called chatter vibration, in the die 10. This phenomenon tends to occur particularly in the case of large cutting depth. The chatter vibration deteriorates the precision of width and depth of the grooves 20, and damages the tool 40.
In the case of machining by down-cutting, when grooves 22 are formed crossing the already machined grooves 20 (first pass), burrs 50 are generated at intersections 23 of the grooves 20 and the grooves 22 as shown in FIG. 6(a). When a rotating tool 40 passes the intersections 23 in the second-pass machining as shown in FIG. 6(b), the burrs 50 are entrained into gaps between the rotating tool 40 and the die-forming work 11, so that the tool 40 is broken or warped. When the tool 40 is broken, its fragments scratch the grooves. Also, the warped tool 40 provides the grooves 20 with partially enlarged width.
JP 11-70510 A discloses a method for producing a die for molding a honeycomb structure having pluralities of apertures for supplying a moldable material, and slit grooves communicating with the apertures and arranged in a lattice pattern to form a honeycomb molding, each slit groove having depth 10 times or more its width, the machining of the above slit grooves being conducted by grinding or cutting a die-forming work with a rotating tool having a thickness of 150 μm or less, and the order of machining pluralities of parallel slit grooves in the same direction being such a random order that the change of the groove width by the machining order does not affect the moldability of the honeycomb structure. It is described that this method prevents the breakage of a tool while suppressing width variation of the slit grooves.
Although the method of JP 11-70510 A is effective to some extent to prevent the meandering of grooves and damage to the tool, a sufficient effect cannot be obtained particularly when the tool has a reduced cutting performance due to wear, failing to solve these problems completely. As a result, the tool is likely broken during machining, its fragments scratching grooves, and when the tool is warped if not broken the groove width partially expands, causing irreparable damage. If the die had scratch or partially enlarged width in a groove even at only one point, an extrusion-molded honeycomb structure would suffer defects such as deformation, breakage, etc. in its cell walls, making the die unusable.
The above problems make the production cost of a molding die higher, posing a large obstacle to the reduction of the production cost of a honeycomb structure. Accordingly, a further improvement is needed.